Industrial processes are used to manufacture various products such as refined oil, pharmaceuticals, paper, foods, other products, or any combination thereof. Such industrial processes are typically controlled using process monitoring and control systems, which use multiple field devices (i.e., process sensors, process actuators, and other electronic devices) to monitor and control operation of the industrial processes. In general, field devices are electronic devices that can be coupled to process equipment to sense a process parameter and to communicate data related to the sensed process parameter to a control system. Such process parameters can include pressure, temperature, fluid flow rate, other process parameters, or any combination thereof. Further, field devices can include actuator circuitry that can be controlled by the control system to adjust a parameter, for example, by adjusting a valve.
One type of field device can be referred to as a pressure transmitter. A pressure transmitter can be coupled to industrial equipment to measure a pressure associated with a process fluid (such as a gas, a liquid, or any combination thereof) and can communicate data related to the measured pressure to a control system, either via a wire or via a wireless communication protocol. Pressure transmitters can be used to measure differential, absolute, or gauge pressures. Further, using known techniques, pressure transmitters can be used to measure flows of the process fluid based upon a pressure differential in the process fluid between two locations.
Typically, a pressure transmitter includes a pressure sensor, which is coupled to a process fluid via an isolation system. The isolation system can include an isolating diaphragm that physically contacts the process fluid. Further, the isolation system can include an isolation fill fluid that extends between the isolation diaphragm and the pressure sensor. The isolation fill fluid is a substantially incompressible fluid such as an oil. As the process fluid exerts a pressure on the isolation diaphragm, changes in the applied pressure are conveyed across the diaphragm and through the isolation fluid to the pressure sensor. Such isolation systems prevent the delicate components of the pressure sensor from being directly exposed to the process fluid and to heat associated with the process fluid.
In flammable environments, a capillary tube is configured to convey the isolation fill fluid between the diaphragm and the sensor. If the diaphragm ruptures, the capillary tube is intended to prevent flames from extending from within the housing through the capillary tube. The International Electrotechnical Commission (IEC) has promulgated an acceptable flame-path configuration for a round shaft that extends through a hole in an enclosure, where the gap between the shaft and the hole is approximately 0.008 inches or less around the circumference of the shaft and where the wall thickness of the enclosure is approximately 0.375 inches or more. The accepted flame path configuration is defined an IEC document number 60079-1 (2003). However, a capillary tube having a cylindrical lumen with a diameter of approximately 0.008 inches can result in delayed pressure measurements from the sensor, because reduced diameter of the capillary tube may impede the flow of the isolation fluid fill due to the viscosity of the isolation fluid fill. Such delayed pressure measurements can result in delayed process control signals, which may delay transmission of control signals to alter a process, resulting in damage to the process equipment.